The present invention relates to a method of increasing the reliability of an inkjet printer, containing at least one pressure chamber provided with a nozzle, wherein ink drops are jetted from the nozzle and a disturbance is detected in the pressure chamber, whereafter the jetting of the ink drops is interrupted. The present invention also relates to an inkjet printer suitable for use with this method.
This method is known from U.S. Pat. No. 4,625,220 and is used to remove disturbances in the pressure chamber which influence the operation of the pressure chamber. In a printer of this kind, a pressure pulse is generated in the pressure chamber by means of a pulse generator. This results in a pressure wave in the ink in the pressure chamber and an ink drop is jetted from the nozzle. Disturbances in the pressure chamber, e.g. a gas bubble or an unwanted solid particle, result in generated pressure waves in the pressure chamber having a deviation from the standard pressure wave, i.e. the pressure wave which precedes the jetting of a regular (on average good) ink drop. Thus a disturbance may result in ink drops having a volume different from the volume of a regular ink drop. It is also possible that a disturbance may result in the presence of one or more disturbing satellite drops at each jetted ink drop. In an extreme case, a disturbance may even result in the breakdown of the pressure chamber, so that it is subsequently impossible to jet ink drops from the nozzle. The method proposes detecting a disturbance in the pressure chamber whereafter the jetting is temporarily interrupted so that print artefacts can be obviated. During the interruption, an active restoration operation is performed, in which the pressure chamber is flushed with new ink so that the old ink, including the disturbance, is removed from the pressure chamber. After the restoration operation has been carried out, ink jetting is resumed.
An important disadvantage of this method is that flushing the pressure chamber with new ink is accompanied by a considerable loss of expensive ink, since after the flushing operation the ink is frequently discharged to a waste container. Although there are methods known to collect the ink with which the pressure chamber has been flushed out and return it to the ink supply of the inkjet printer, these methods are combined with complex cap constructions since care must be taken at all times to prevent dirt, dust, air or other impurities reaching the ink from the exterior, since they can in turn result in disturbances in the pressure chamber. Moreover, in the latter case a return system is required, which is complex particularly in the case of meltable inks, since these inks solidify shortly after they have left the (heated) pressure chamber. In addition, due to the miniaturization of the print heads of inkjet printers, the caps are often many times larger than the dimensions of one nozzle, so that when one pressure chamber is flushed a number of nozzles of any pressure chambers in the vicinity are also flushed with ink, and this means a further waste.
Another significant disadvantage of such active restoration operations is that the entire print head to which the pressure chamber belongs cannot be used for printing substrates during the flushing operation, so that the inkjet printer productivity is under considerable pressure if high reliability is required.
The method according to the present invention is intended to obviate these disadvantages. To this end, a method has been developed in which the jetting is interrupted for a predetermined time. During this time no active restoration operations are carried out to remove the disturbance, and the pressure chamber is simply left to itself. After the predetermined time has elapsed, jetting of the ink drops from the nozzle of the pressure chamber is resumed. This method is based on the recognition that practically all disturbances disappear by themselves if the pressure chamber is not activated for a specific time. This method has the considerable advantage that there is no need to flush the pressure chamber with new ink in order actively to remove the disturbance from the pressure chamber. As a result no ink is wasted in removing the disturbance. Another important advantage is that any other pressure chambers in the print head need not interrupt the jetting, so that a print job started can be continued.
The temporary non-activation of the pressure chamber in which a disturbance is present may lead to very small artefacts in a printed image, which are practically invisible to the observer, and if considered necessary they can be intercepted in the manner known to the skilled man, for example in the manner known from Japanese Patent 60-104335. By the use of such a method there is practically no negative effect on the inkjet printer productivity and the temporary interruption of the jetting of the pressure chamber is prevented from resulting in print artefacts in the printed image. Another advantage of the method according to the present invention is that it is already known in advancexe2x80x94i.e. directly prior to the actual interruption of jettingxe2x80x94when jetting will be resumed, since resumption is not dependent on an active restoration operation being completed. This advantage can be used, inter alia, in determining the most optimal print strategy.
In a preferred embodiment, directly prior to the disturbance, a predetermined number of pressure pulses is generated following the detection of a previous disturbance. It has been found that most of the pressure waves which differ from the standard pressure wave do not lead to visible print artefacts. If jetting of the pressure chamber were temporarily interrupted after the occurrence of such a disturbance, it would result in an unnecessary fall-off in the inkjet printer productivity. Consequently, it is advantageous to determine which disturbances form a risk to the operation of the pressure chamber, and to interrupt the jetting of that chamber only when such disturbances are present.
It has now been found that a disturbance practically certainly leads to visible print artefacts if the disturbance does not disappear during the jetting of a specific number of ink drops from said pressure chamber. The reason why most disturbances disappear spontaneously during the jetting of a specific number of ink drops and that a small number of the disturbances increases precisely during said jetting is not completely clear. It may be that most disturbances are small gas bubbles which dissolve spontaneously in the ink before they reach a size such as to be accessible to growth under the influence of the pressure pulses. Another reason might be that disturbances occur mainly in the neighborhood of the nozzle with which they are jetted with the ink drops from the pressure chamber before they can lead to perceptible print artefacts. In this preferred embodiment, after it has been found that a disturbance is present in the pressure chamber, a specific number of pressure waves is generated in the associated pressure chamber in order to jet ink drops, whereafter it is determined whether there is still a disturbance present. If not, then the disturbance has obviously disappeared and there is no further acute risk to the operation of the pressure chamber. If the disturbance is still present, then there is a considerable chance that the last detected disturbance has or will have a perceptible negative effect on the function of the pressure chamber, so that visible print artefacts will occur in a printed image. To obviate this, jetting is temporarily interrupted and the pressure chamber is left to itself for a specific time.
In another preferred embodiment, a maximum of 100 pressure pulses is generated in the pressure chamber between the previous and the later disturbance. By making the number of pressure pulses not more than 100, the disturbance is prevented from becoming too large, so that it would take too long before it disappears when jetting is interrupted. In another preferred embodiment, the number of pressure pulses is smaller than or equal to 50. In yet another preferred embodiment said number is smaller than or equal to 20. This number is always predetermined, and hence known at the time when a start is made on generating the first pressure pulse of said number, but can be adapted to the type of machine, the ink used, the machine history (wear), the magnitude of the disturbance, and so on. In another preferred embodiment of the method according to the present invention, the inkjet printer comprises at least a first and a second pressure chamber, and after detection of a disturbance in the first pressure chamber the jetting of ink drops from the nozzle of said pressure chamber is interrupted for a predetermined time but the jetting of ink drops from the nozzle of the second pressure chamber is continued during the interruption. In this way, considerable reliability of the inkjet printer is combined with a minimal loss of productivity.
In order to be able to accurately determine whether a disturbance is present in the pressure chamber, the latter is preferably provided with an electromechanical transducer (piezo-element), a drive circuit with a pulse generator to activate the transducer, and a measuring circuit such that the disturbance is detected by measuring the electrical signal generated by the transducer in response to an activation, by means of the measuring circuit. In a piezo-inkjet printer of this kind, the transducer is activated by generating an electrical pulse with the pulse generator which forms part of the drive circuit. In this way the transducer is subject to deformation so that the pressure wave is generated in the pressure chamber and accordingly an ink drop is jetted from the nozzle. The pressure wave generated in turn deforms the electromechanical transducer so that it generates an electrical signal. By measuring this signal with the measuring circuit it is possible to determine whether a disturbance is present in the pressure chamber since a disturbance will result in a deviation in the generated pressure wave. In this way, the transducer, the primary task of which is to generate pressure waves in the pressure chamber, is also used as a sensor. It has been found that by measuring the electrical signal generated by the transducer because the latter is in turn subjected to mechanical deformation by the pressure wave which it has generated in the pressure chamber, it is possible accurately to determine the magnitude of the deviation of a pressure wave from a standard pressure wave. In another preferred embodiment, the drive circuit is opened when the measuring circuit is closed. This further improves detection of the deviation of a pressure wave generated in the pressure chamber by activating the transducer. The present invention also relates to an inkjet printer suitable for use with the present method. The inkjet printer is preferably a piezo-inkjet printer. Also, preferably, a meltable ink, i.e. a hot-melt ink, is used in the printer.